Nitrogen-based atmospheres have been routinely used by the heat treating industry both in batch and continuous furnaces since the mid-nineteen seventies. Because of the low dew point and virtual absence of oxygen, nitrogen-based atmospheres do not exhibit oxidizing and decarburizing properties and are therefore suitable for a variety of heat treating operations. More specifically, a mixture of nitrogen and hydrogen has been extensively used for annealing of low to high carbon and alloy steels, annealing of non-ferrous metals and alloys such as copper, copper alloys, gold alloys, and sintering metal and ceramic powders. Mixtures of nitrogen and a hydrocarbon gas such as methane and propane have gained wide acceptance for neutral hardening and decarburized-free annealing of medium to high carbon steels. Nitrogen and methanol mixtures have been developed and used for carburizing low to medium carbon steels. Finally, a mixture of nitrogen, hydrogen, and/or moisture has been used for brazing metals and sealing glass to metals.
A portion of nitrogen used by the heat treating industry is produced by distillation of air in large cryogenic plants. Likewise, a portion of hydrogen used by the heat treating industry is produced by either partial oxidation or steam reforming of natural gas. Both nitrogen and hydrogen produced by these techniques are generally very expensive. Furthermore, the nitrogen-hydrogen atmospheres required for a variety of annealing, heat treating, brazing, sealing, and sintering operations and prepared by blending these gases are also very expensive. To reduce cost, a large number of heat treaters have been producing nitrogen-hydrogen atmospheres by decomposing (or cracking) ammonia in ammonia dissociators. Ammonia dissociators located remotely have been employed in some cases to generate nitrogen-hydrogen atmospheres for a variety of heat treating operations. In other cases, ammonia dissociators have been integrated with furnaces to save floor space and to improve overall thermal efficiency.
Ammonia dissociators generally decompose ammonia into a mixture of nitrogen and hydrogen over a bed of nickel, iron, or nickel/iron catalyst supported on a ceramic material. U.S. Pat. Nos. 3,598,538, 3,379,507, 4,179,407 disclose details of ammonia dissociators. The catalyst normally promotes the following ammonia dissociation reaction: EQU 2NH.sub.3 =N.sub.2 +3H.sub.2
This reaction is endothermic and requires heating of the catalyst bed from an outside source generally to temperatures ranging from 600.degree. C. to 950.degree. C. Operating pressure in the unit generally ranges from 2 psig to 100 psig, and the space velocity used for the dissociation reaction generally varies from 500 to 5,000 Nm.sup.3 /h product gas per m.sup.3 of the catalyst. The product gas generally contains a mixture of 25% nitrogen and 75% hydrogen with small quantities of residual ammonia measured in PPM. Since the dissociation reaction is correlated to the thermodynamic equilibrium, the content of unconverted ammonia in the product gas can vary from 30 ppm to 500 ppm depending on the operating temperature, pressure, and space velocity.
The concentration of hydrogen in nitrogen-hydrogen atmospheres required for the majority of heat treating operations generally varies from about 0.5 to about 25%. Since cryogenically produced nitrogen is cheaper than nitrogen-hydrogen atmosphere produced by dissociating ammonia, heat treaters blend nitrogen with dissociated ammonia product gas to reduce overall atmosphere cost and to produce nitrogen-hydrogen atmosphere with the desired composition. However, these heat treaters are still experiencing the dilemma of high nitrogen-hydrogen atmosphere cost. Furthermore, it is increasingly becoming difficult for them to compete effectively in the open world market.
Since the concentration of nitrogen in nitrogen-hydrogen atmospheres varies from about 75% to 99.5%, it is conceivable to reduce the overall cost of nitrogen-hydrogen atmospheres by using low-cost nitrogen produced by non-cryogenic air separation techniques such as pressure swing adsorption (PSA) and selective permeation (membrane separation). The non-cryogenically produced nitrogen costs less to produce, however it contains from 0.05 to 5.0% residual oxygen, making a direct substitution of cryogenically produced nitrogen with non-cryogenically produced nitrogen very difficult.
Furnace atmospheres suitable for heat treating applications have been generated from non-cryogenically produced nitrogen by removing residual oxygen or converting it to an acceptable form in external units prior to feeding the atmospheres into the furnaces. Such atmosphere generation methods have been described in detail in French publication numbers 2,639,249 and 2,639,251 dated 24 November 1988 and Australian patent application numbers AU45561/89 and AU45562/89 dated 24 Nov. 1988. These methods require use of external units packed with expensive precious metal catalysts such as palladium and platinum. The use of an external unit considerably increases the cost of noncryogenically produced nitrogen and that of nitrogen-hydrogen atmosphere. Thus, heat treaters with continuous furnaces equipped with ammonia dissociators have not converted to non-cryogenically produced nitrogen.
It is clear that there is a need to switch from cryogenically produced nitrogen to non-cryogenically produced nitrogen for reducing the overall cost of nitrogen-hydrogen atmospheres for heat treating in continuous furnaces equipped with ammonia dissociators.